template function why can't add “cin”?

Question

#include <iostream>
using namespace std;

template<int x, int y> 
void add()
{
    cin >> x >> y;
    cout << x + y << endl;
}

int main()
{
    add<1,2>();

    return 0;
}

In Windows10 + visual studio 2017 , it gets an error : Binary > > : the operator of the left operands of the STD: : istream type is not found (or there is no acceptable conversion)

The parameter x and y is something different from other normal int variables?

Answer 1

I think you want something more like this:

#include <iostream>
using namespace std;

template<class T>
void add(T x, T y)
{
    cin >> x >> y;
    cout << x + y << endl;
}

int main()
{
    add(1, 2);

    return 0;
}

In your example, x and y are template parameters, yet you're trying to use them as values in your cin and cout statements.

Answer 2

Yes, template parameters are different than normal function parameters. Template parameters are compile time constants. Given your definition of the add template, when you instantiate it with add<1,2> , the compiler essentially creates a function like this:

// where 'function_name' is a compiler generated name which is
// unique for the instantiation add<1,2>
void function_name()
{
    cin >> 1 >> 2;
    cout << 1 + 2 << endl;
}

Obviously, you can't do this:

cin >> 1 >> 2;

You need actual modifiable objects to input into, not constants.